Quantifying the Impact of the Unvaccinated

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, March 18, 2016

The year 2014 was a remarkable one for infectious diseases. The Ebola outbreak exploded, chikungunya spread through the Western hemisphere, MERS and avian influenza continued their steady march, and the US experienced a large measles outbreak centered at Disneyland. Of these infectious diseases, only 1 is vaccine preventable: measles.

The rise of the antivaccine movement has received much attention and has sparked policy debates in many states. These debates are best informed by rigorous scientific studies that address the impact of the unvaccinated on disease spread. A new study from Saad Omer, a researcher who studies vaccine-preventable diseases, provides important insights into this field.

Measles: The Impact of Vaccination

To assess the impact of vaccination on measles, Omer and colleagues surveyed the medical literature on the incidence of measles in the US (post-elimination in the US in 2000). The results of the literature review revealed that 1,416 cases of measles have been reported in the US since 2000. Of these cases, 56.8% were in individuals with no record of measles vaccination. In those studies in which information was available (970 of the cases), over 70% of those who were unvaccinated relied on nonmedical exemptions. One study revealed that the unvaccinated, who are a substantial part of the first generation of an outbreak, are 35 times more likely to contract measles than a vaccinated individual.

Pertussis: Waning Immunity Plays a Role

Outbreaks of pertussis in the US were studied post-1977, the year the US reached its nadir of cases with just 1,010. The literature reviewed yielded 10,609 cases of pertussis for which vaccination status was reported. Though some outbreaks revealed that a substantial proportion of cases were unvaccinated or undervaccinated, many cases occurred in highly vaccinated populations, attesting to the impact of waning pertussis immunity (in some cases due to the less potent acellular vaccine). Additionally, states in which personal vaccination exemptions are available had higher rates of pertussis outbreaks.

School Vaccination Policy Should Reflect Science

The implications of this study cannot be overstated. The gains made against infectious diseases in the US are the result of high vaccination coverage. However, the falling incidence of vaccine-preventable diseases has engendered complacency and, when combined with the vociferous advocacy of the antivaccine movement, has created a perfect storm for the recurrence of measles and, to a lesser but still significant extent, pertussis.

As exemption policies for public schools are evaluated and debated, understanding the impact of nonmedical exemptions (ie, religious and philosophical exemptions) on the burden of illness is essential. Studies like Omer’s provide an important scientific framework that should be reflected in the policy conversation.